Cardiac hypertrophy and progression to overt heart failure is associated with significant morbidity and mortality. Mammalian models of cardiovascular disease are tremendously helpful to study cardiac physiology and signaling pathways, however; these models are not readily amenable to large genetic screens. Therefore, strategies using Drosophila as model to identify new genes that cause or modify cardiomyopathies were developed to circumvent some of the complexities associated with mammalian models of cardiovascular disease. The epidermal growth factor receptor (EGFR) is necessary to maintain normal post- developmental cardiac function in adult Drosophila. Moreover, the activation of EGFR and subsequent signals to the small GTPase, Ras, and the kinases, Raf and MEK1, cause cardiac hypertrophy in flies and mammals. Signaling through the kinase, Hippo, alters the activity of the co-transcriptional activator, yorkie, and also produces cardiac hypertrophy similar to the EGFR/Ras/Raf pathway in flies. The long-term goal is understand the molecular signals that modulate the development and progression of cardiac hypertrophy and dilated cardiomyopathy using Drosophila genetics as a discovery platform and translate the findings to humans. The overall objective for this application is to examine the Raf-MEK1 pathway and the activation of yorkie in cardiac hypertrophy. The central hypothesis is that crosstalk between the Raf-MEK1 and Hippo-YAP1 pathways drives cardiomyocyte decisions towards hypertrophy or hyperplasia. The following Specific Aims will be examined to test the central hypothesis: (1) Determine whether Raf-MEK1-mediated cardiac hypertrophy occurs via regulation of the Hippo- yorkie pathway in flies and identify signaling pathways that function downstream of yorkie.; (2) Identify the specific modular domains in mammalian YAP1 that drive cardiomyocyte hypertrophy or proliferation; and (3) Determine whether a reduction in YAP-1 modifies the severity of cardiac hypertrophy or heart failure in RafL613V/+ knock-in mice or wild-type mice after TAC-induced pressure overload. This proposal is innovative in its use of Drosophila to rapidly identify novel molecules in two evolutionarily conserved signaling pathways and translate these findings to mammals. The proposal is expected to identify previously unknown components that modify Raf-dependent cardiac hypertrophy and lead to better understanding of pathways that drive pathologic hypertrophy in humans.